1. Field of the Invention
The present invention relates to a device for applying time-delays to optical signals, in particular to signals conveyed by an optical data transmission system.
2. Description of the Prior Art
In some applications in optical telecommunication systems it is important to be able to apply time-delays to optical signals precisely and continuously within a very small dynamic range. For example, the range for binary signals at a bit rate of 40 Gbit/s has a duration of approximately 25 ps when it is necessary to be able to adjust a time-delay to a duration of the order of one bit period.
Time-delays with such characteristics are useful for converting wavelength division multiplexed (WDM) signals into time division multiplexed (TDM) signals, for example.
Another application is to regenerate WDM signals by resynchronizing the spectral channels before synchronous reshaping modulation is applied by a common modulator.
On the application of time-delays, see for example the articles by N. V. JESPERSEN, A. C. HEATH and E. S. ROLLER, SPIE, vol 756, p 156 (1987) and P. R. HERCZFELD et al., xe2x80x9cWide-band true time-delay phase shifter devicesxe2x80x9d, Proc. IEEE MTT-5 International Microwave Symposium Digest, Las Vegas, Nev., Jun. 1987, pp 603-606, which describe delay lines in which the time-delay can be changed by stretching the time-delay fiber.
Wrapping an optical fiber onto a piezo-electric mandrel provides an electrical way to control the length of the fiber. Relative variations of length of the order of 10xe2x88x923 can be obtained by this method, enabling a time-delay of 100 ps to be obtained with a few tens of meters of fiber. A delay line of the above kind therefore provides continuous time-delays.
The above solution nevertheless has a number of drawbacks: the high control voltages needed (of the order of 1 kV), the hysteresis of piezo-electric stretching, and the fact that the response time is long because the time-delay control bandwidth is limited by the mechanical inertia of the mandrel (to at best a few tens of kilohertz). A delay line of the above kind also consumes a large amount of energy under dynamic conditions, due to the high capacitance of piezo-electric elements. A device of the above kind is also heavy, because it comprises not only the necessary length of fiber but also the piezo-electric mandrel.
Another prior art solution uses a thermally controlled delay line with a typical sensitivity of 50 ps/xc2x0 C.km. A 25 ps range of time-delays can be covered with a fiber 50 m long and a thermostatically controlled device whose temperature can be varied within a range of 10xc2x0 C.
However, this solution has a high inertia and is subject to serious temperature stabilization problems.
The present invention aims to alleviate the problems of the aforementioned devices by proposing a device for applying a time-delay that is easy to use and accurate and has a very low inertia.
A supplementary object of the invention is to provide a device whose performance is only slightly dependent on the polarization of the optical signals supplied to the input.
Accordingly, the invention provides a device for applying a time-delay to optical signals taking the form of modulation of a carrier wave having a center wavelength, which device includes:
a first phase modulator adapted to receive an input optical signal carried by an original center wavelength and to apply a first stage of phase modulation to the carrier wave of the input signal to supply a first intermediate signal carried by a modified center wavelength,
a delaying dispersive member having chromatic dispersion and adapted to receive the first intermediate signal and to supply a second intermediate signal, and
a second phase modulator adapted to receive the second intermediate signal and to apply a second stage of phase modulation to the carrier wave of the second intermediate signal to supply an output signal carried by the original center wavelength.
The device according to the invention can have one or more of the following features:
it further includes a control unit for the first and second phase modulators adapted to adjust the depths of phase modulation respectively applied to the input signal and to the second intermediate signal;
the input signal is a binary signal having a particular bit period and the control unit is adapted to control the first and second phase modulators periodically with a period equal to the bit period;
the maximum depth of phase modulation respectively applied to the input signal and to the second intermediate signal and/or the chromatic dispersion of the delaying dispersive member are chosen to obtain a range of time-delays which is at least equal to the bit period;
it further includes means for modifying the chromatic dispersion of the delaying dispersive member.
each of the first and second phase modulators is adapted to apply to the optical signals phase modulations which are substantially independent of the state of polarization of the signals.
Thus the invention exploits the property of dispersive media, such as dispersive fibers or fibers with a photo-written Bragg grating, of imposing on an optical wave passing through them a propagation speed which is dependent on the wavelength (or the optical frequency) of the wave. Because of the first phase modulator, the modification applied to the wavelength of the wave carrying the pulses constituting an input optical signal can be adjusted to vary the propagation time of those pulses in the delaying dispersive medium. By opposite phase modulation the second phase modulator returns the wavelength to its original value.
Note that this solution is very suitable for optical signals with RZ amplitude modulation because the phase modulation can be applied very simply using a clock signal at the bit frequency. For other types of modulation, such as NRZ amplitude modulation, there are additional constraints with regard to the phase modulators because the maximum phase variation to be applied is greater.
For the standard signals referred to above, it is known in the art that dispersive media widen the optical pulses. The presence of a dispersive member in the device according to the invention must also be taken into account in some situations in which the chromatic dispersion of the delaying dispersive member must be high to provide a wide range of time-delays. Compensation is required in particular if the pulses at the output of the delaying dispersive medium have widths which are incompatible with the phase modulation applied by the second modulator because of chromatic dispersion.
The chromatic dispersion coefficient D of a medium is related to its propagation constant xcex2 by the equation:
d2xcex2/dxcfx892=xe2x88x92(2xcfx80c/xcfx892)D, 
in which xcfx89 is the angular frequency of the optical wave and c is the speed of light in a vacuum.
As a general rule, the coefficient D can be positive, zero or negative according to the wavelength and the medium used. For standard fibers, for example, the chromatic dispersion is approximately +17 ps/(km.nm) at a wavelength of 1.5 xcexcm.
A chromatic dispersion value is defined for a homogeneous or non-homogeneous dispersive member, for example a link incorporating a dispersive fiber, which can be expressed mathematically by the following equation:                     DL        =                  ∫                                    D              ⁡                              (                z                )                                      ·                          ⅆ              z                                                          (        1        )            
where z is the abscissa of a point along the dispersive medium, D(z) is its chromatic dispersion parameter at abscissa z, and the integral which expresses the dispersion DL is calculated along the propagation path of waves in the dispersive medium.
Likewise, if a link consists of a plurality of dispersive members connected in cascade, a cumulative chromatic dispersion can be defined for the link as the algebraic sum of the chromatic dispersions of the various members that form the link.
To solve the problem of possible widening of the optical pulses referred to above, the device in accordance with the invention can further include a second dispersive member disposed to derive the input signal of the first phase modulator from an optical signal to be delayed, the second dispersive member having a chromatic dispersion of opposite sign to the chromatic dispersion of the delaying dispersive member and whose absolute value is less than that of the delaying dispersive member.
This ensures that the absolute value of the cumulative chromatic dispersion evaluated on the basis of the signal to be delayed up to each of the phase modulators remains below the absolute value of the chromatic dispersion of the delaying dispersive member.
Of course, the foregoing arrangements imply that the pulses of the signal to be delayed are not widened by a dispersive member upstream of the device, i.e. a dispersive transmission link such as a standard fiber.
If this is not the case, the device according to the invention includes a second dispersive member disposed to derive input signal to the first phase modulator from a signal emitted by an optical link, the second dispersive member having a chromatic dispersion such that the cumulative chromatic dispersion of the optical link and the second dispersive member is of opposite sign to the chromatic dispersion of the delaying dispersive member and the absolute value of the cumulative chromatic dispersion is less than that of the delaying dispersive member.
The second dispersive member is essential only if the cumulative chromatic dispersion upstream of each phase modulator, evaluated on the basis of the signal to be delayed or the signal transmitted, is sufficient to widen the pulses significantly. The second dispersive member can also be dispensed with if the input signal consists of a stream of soliton pulses and the chromatic dispersion of the delaying dispersive member is positive.
The invention further provides a converter for converting wavelength division multiplexed optical signals into time division multiplexed optical signals including at least one device as defined above.
Other advantages and features of the invention will become apparent on reading the following description, which is given by way of non-limiting example and with reference to the accompanying drawings.